ABSTRACT,8-Amylase of sweet potato (Ipomoea batatas L.), which constitutes about 5% of the total soluble protein of the tuberous root, is absent or is present in only small amounts in organs other than the tuberous roots of the normal, field-grown plants. However, when leaf-petiole cuttings from such plants were supplied with a solution that contained sucrose, the accumulation of 8-amylase was induced in both leaf and petiole portions of the explants. The sucrose-induced accumulation of fl-amylase in leaf-petiole cuttings occurred concomitant with the accumulation of starch and of sporamin, the most abundant storage protein of the tuberous root. The accumulation of fl-amylase, of sporamin and of starch in the petioles showed similar dependence on the concentration of sucrose, and a 6% solution of sucrose gave the highest levels of induction when assayed after 7 days of treatment. The induction of mRNAs for f-amylase and sporamin in the petiole could be detected after 6 hours of treatment with sucrose, and the accumulation of j6-amylase and sporamin polypeptides, as well as that of starch, continued for a further 3 weeks. In addition to sucrose, glucose or fructose, but not mannitol or sorbitol, also induced the accumulation of fl-amylase and sporamin, suggesting that metabolic effects of sucrose are important in the mechanism of this induction. Treatment of leaf-petiole cuttings with water under continuous light, but not in darkness, also caused the accumulation of small amounts of these components in the petioles, probably as a result of the endogenous supply of sucrose by photosynthesis. These results suggest that the expression of the gene for f-amylase is under metabolic control which is coupled with the expression of sink function of cells in the sweet -otato.

Sporamin, the major soluble protein of the sweet potato tuberous root, is coded for by a multigene family. Fourty-nine essentially full-length sporamin cDNAs isolated from tuberous root cDNA library have been classified by cross hybridization, restriction endonuclease cleavage pattern and ribonuclease cleavage mapping. All the cDNAs fall into one of the two distinct homology groups, subfamilies A and B, which correspond to the polypeptide classes sporamin A and B, respectively. At least 5 different sequences are detected in both of the 22 sporamin A and 27 sporamin B cDNAs. Comparison of the nucleotide sequences of the coding region of three each of sporamin A and B subfamily members, four from cDNAs and two from genomic clones, indicates that intra-subfamily homologies (94 to 98%) are much higher than inter-subfamily homologies (82 to 84%), and there are deletions or insertions of one or two codons at three locations which characterize each subfamily. Large portions of base substitutions in the coding region accompany amino acid substitutions. In contrast to the coding region, most of the structural differences among the members in the 5' and 3' noncoding regions are deletions or insertions.

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